Graft copolymers having a core/shell structure composed of an organosilicon polymer component and an organic polymer component are described in a number of publications.
DE-B 1595554 (U.S. Pat. No. 3,445,415) discloses a process for preparing aqueous graft polymer latices in which monoolefinically unsaturated monomers are grafted onto organosiloxane polymers comprising units of the general formula RSiO.sub.3/2. In this process only hard polymers and not graft copolymers having elastomeric properties can be prepared.
DE-B 2421288 (U.S. Pat. No. 3,898,300) describes a process for preparing graft copolymers in which styrene and other monoethylenically unsaturated compounds are grafted onto a polyorganosiloxane graft base. To this end, emulsions containing mixtures of polyorganosiloxanes or mixtures of polyorganosiloxanes and organosiloxanes are homogenized using a homogenizer and subsequently grafted with the organic monomers. In this complex process, only polydispersed graft copolymer dispersions having a broad particle size distribution can be obtained. The preparation of graft copolymers having a monomodal particle size distribution and having particle sizes of &lt;0.1 .mu.m is not possible using this process.
Graft copolymers comprising undefined organopolysiloxane or silicone rubber and vinyl or acrylic monomers are described in DE-A 2539572. In the polymerization, the mixture is stirred using high-speed stirrers. A polydispersed product having particle sizes of between 1 and 3 mm is obtained.
DE-A 3629763 (EP-A 258746) describes silicone rubber graft copolymers with vinyl or acrylic monomers, in which the silicone rubber phase is at least partially crosslinked. Although, in the preparation of the graft base, the batch is already homogenized, the particle size of the graft base is already 300 nm. Homogenization results in a polydispersed particle size distribution.
EP-A 254418 relates, inter alia, to silicone graft copolymers obtained by grafting acrylates onto dispersed silicone elastomers. The particle size of the graft copolymers obtainable in this manner is 0.14 .mu.m. Monomodal graft copolymers are not described.
EP-A 231776 describes a mixture containing polyester and polysiloxane graft copolymers. The polysiloxane is prepared by emulsion polymerization of the monomeric silanes after prior homogenization using an Ultraturrax or a homogenizer. The polysiloxane graft base is subsequently grafted with vinyl monomer. The polyorganosiloxane graft copolymers described in U.S. Pat. No. 4,690,986 are prepared by the same method. The particle size of the graft copolymers in the Examples is 300 nm due to the homogenization. A polydispersed particle size distribution is obtained.
Particulate graft copolymers having a core/shell structure and containing polysiloxanes or silicones and more than one shell are described in DE-A 3617267 (EP-A 246537), DE-A 3631539 (U.S. Pat. No. 4,812,515), EP-A 296402 (U.S. Pat No. 4,865,917) and in EP-A 296403 (U.S. Pat. No. 4,885,209). The siloxane or silicone rubber graft base is prepared in all cases after a homogenization step, which results in a polydispersed particle size distribution.
DE-A 3617267 and DE-A 3631539 describe a graft copolymer having a silicone rubber core, a first shell of acrylate rubber and a grafted-on shell of monoethylenically unsaturated monomers.
EP-A 296402 and EP-A 296403 relate to silicone rubber graft copolymers comprising a rubber-like organopolymer core having a shell of organopolysiloxane, onto which ethylenically unsaturated monomers are grafted.
Therefore, it is an object of the present invention to provide finely divided elastomeric graft copolymers containing organosilicon and organic polymers and having a monomodal particle size distribution. Another object of the present invention is to provide a process for preparing graft copolymers which does not utilize any complex mechanical emulsification or homogenization steps, and can be used to modify the particle size without using additional emulsifier. A further object of the present invention is to provide a process for preparing graft copolymers in which grafting takes place to an adequate extent, even without a functional silane component in the core.